The outcry caused by events in the news in which persons have been held under house arrest, special surveillance and held on leave or in semi-liberty has had the effect of concentrating the attention of public opinion on electromagnetic devices for personal location, giving rise to a debate in which the arguments of the sceptics are losing ground and the adoption of such a system seems to be settled.
From reading the considerable amount of material published in the media, it is clear that many products, obviously foreign made, are already available, based on many different technologies and with many differing requirements.
The choices of technology made by the present manufacturers are highly diverse and some can certainly be criticised either from a technical point of view or for their functional logic.
It is possible to divide these devices, now known as “bracelets”, into two groups.                Geographical location systems, designed to determine the position of the detainee wherever he may be.        Area location systems, able to reveal the presence of the detainee within a predetermined area.        
Geographical location systems are based on various principles, among which the most modern and effective consists of a combination of GPS (satellite positioning system) and GSM (digital cellular telephony), which permits precise determination of the geographical coordinates of the detainee which are then transmitted to an operations centre where the coordinates can be transferred onto an electronic map for immediate display.
Such systems, that on first analysis might appear to be the definitive solution to the problem, are in reality extremely vulnerable because of satellite coverage, which can be easily screened outdoors and is non-existent inside buildings; also the GSM link is unreliable, as is well known to its users, therefore to entrust such a delicate subject to this technology is, on further analysis, imprudent.
Moreover, however strong it is possible to make the bracelets, it will always be possible to succeed in getting rid of them, making knowledge of their position useless.
From a strictly functional point of view, it is not necessary geographically to locate a person who automatically becomes a fugitive if he is not in the area stipulated for his house arrest. Furthermore, detainees who have the ability to sustain a lengthy absence are not normally sentenced to house arrest but rather persons whom it is useful to prevent from making occasional escapes, during which he might commit crimes.
Area location systems may be based on various principles, all using a bracelet worn by the detainee that is able to be detected by a fixed base that in turn has links with an operations centre from which countermeasures are coordinated based on the signals received from the fixed equipment.
The technologies upon which area systems are based are generally simple, reliable and well established from their widespread use in security systems and remote alarms, as used in museums, banks, dwellings, etc.
Both the installation and operating cost of these systems are much less and certainly more compatible with the high production quantities assumed.
These systems lend themselves to the adoption of various operating scenarios, choosing between whether the local control unit should be active or passive and whether it is periodically called by the operations centre or calls only in case of necessity, significantly cutting the costs of connection.